This invention relates to lower alkane oxidative dehydrogenation catalysts and a production process of olefins using said catalysts. More specifically, the invention relates to the catalysts which are suitable for use in vapor phase oxidative dehydrogenation of C2-C5 lower alkanes (hereinafter occasionally referred to simply as xe2x80x9clower alkanesxe2x80x9d) in the presence of molecular oxygen to produce corresponding olefins, and a process for oxidizing and dehydrogenating lower alkanes with molecular oxygen to produce corresponding olefins at high yields, with the use of said catalysts.
The invention also relates to a process for producing, from the olefins which have been obtained through vapor phase oxidative dehydrogenation of C2-C5 lower alkanes in the presence of molecular oxygen, the corresponding unsaturated aldehydes and/or unsaturated carboxylic acids.
As a production process for lower olefins, in particular, propylene and isobutene, simple dehydrogenation process of lower alkanes is recently reduced to industrial practice. However, this process is subject to an essential problem that it is incapable of giving high conversion due to the equilibrium limitation and furthermore requires high temperatures. Still in addition, deterioration of the catalyst within a short period is inavoidable in said process, which necessitates frequent regeneration of the catalyst using a switch converter or the like. In consequence, plant construction costs and utility costs for running the process are high and, depending on the conditions of location, it is unprofitable and its industrial application is restricted.
Whereas, attempts to produce lower olefins from lower alkanes through oxidative dehydrogenation which is free from the limitation by equlibrium have been made since long, and various catalyst systems therefor have been proposed. Among those known, there are Coxe2x80x94Mo oxide catalyst (U.S. Pat. No. 4,131,631), Vxe2x80x94Mg oxide catalyst (U.S. Pat. No. 4,777,319), Nixe2x80x94Mo oxide catalyst (EP 379,433 A1) CeO2/CeF3 catalyst (CN 1,073,893A), Mgxe2x80x94Mo catalyst [Neftekhimiya (1990), 30(2) 207-10], V2O5/Nb2O5 catalyst [J. Chem. Commun. (1991) (8) 558-9], rare earth vanadates catalyst [Catal. Lett. (1996), 37, (3, 4), 241-6] and B2O3/Al2O3 catalyst [ACS Symp. Ser. (1996), 638 (Heterogeneous Hydrocarbon Oxidation) 155-169). Those known catalysts, however, invariably show very low level oxidative dehydrogenation performance, the property of the prime importance, and are far short of industrial practice.
Japanese Laid-open (KOKAI) Patent Application, KOKAI No. 245494/1996 furthermore contains a disclosure on a process for further oxidizing propylene, which was formed through dehydrogenation of propane, to produce acrylic acid. This process, however, necessitates removal of the hydrogen formed during the dehydrogenation of propane from the reaction gas. Japanese KOKAI Nos. 045643/1998, 118491/1998, 62041/1980 and 128247/1992, etc. disclose processes for forming unsaturated aldehydes and/or acids from lower alkanes, in particular, acrolein and/or acrylic acid from propane and methacrolein and/or methacrylic acid from isobutane. However, yield of these object products indicated in these publications are very low, and the processes need to be improved in various aspects including the catalyst to be used.
An object of this invention is to provide novel oxidative dehydrogenation catalysts useful for vapor phase oxidative dehydrogenation of lower alkanes with molecular oxygen to produce corresponding lower olefins at high yield; and also to provide a process for producing from lower alkanes the corresponding olefins at high yield, by the use of said catalysts.
Another object of the invention is to provide a process for producing from lower alkanes corresponding unsaturated aldehydes and/or unsaturated carboxylic acids at high yield.
We have made concentrative studies in search of the catalysts suitable for oxidizing and dehydrogenating lower alkanes with molecular oxygen to produce the corresponding lower olefins, to discover that a catalyst containing manganese as the indispensable component, or a catalyst in which said catalytically active component is supported on a refractory inorganic carrier exhibit excellent oxidative dehydrogenation performance; and that lower olefins could be produced at high yield with the use of said catalyst. The present invention has been completed based on these discoveries.
Thus, the present invention provides catalysts for oxidative dehydrogenation of lower alkanes, said catalysts being suitable for use in vapor phase oxidative dehydrogenation of C2-C5 lower alkanes in the presence of molecular oxygen to produce corresponding olefins and characterized by having a composition expressed by a general formula (I) below:
Mnxcex1E1xcex2E2xcex3Oxxe2x80x83xe2x80x83(1)
(in which Mn denotes manganese, and O, oxygen; E1 is at least one element selected from the group consisting of P, As, Sb, B, S, Se, Te, F, Cl, Br, I, Nb, Ta, W, Re and Cu; E2 is at least one element selected from the group consisting of Cr, Fe, Co, Ni, Ag, Au, Zn, Tl, Sn, Pb, Bi, Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Y, La, Ce, Nd and Sm; and xcex1, xcex2, xcex3 and x denote atomic numbers of Mn, E1, E2 and oxygen, respectively, where when xcex1=1, xcex2=0.01-10, xcex3=0-5, and x is a numerical value determined by the state of oxidation of those elements other than oxygen).
The present invention furthermore provides a process for producing olefins which comprises vapor phase oxidative dehydrogenation of C2-C5 alkanes in the presence of molecular oxygen to form corresponding olefins, characterized by the use of the above-described catalyst.
According to the present invention, furthermore, a process for producing, from lower alkane, unsaturated aldehyde and unsaturated acid at high yield is provided, in which an olefin obtained through vapor-phase oxidative dehydrogenation of C2-C5 lower alkanes in the presence of molecular oxygen using the above-defined catalyst is further oxidized at vapor phase in the presence of oxygen to provide unsaturated aldehyde and unsaturated acid.
The invention moreover provides a process for producing unsaturated acid from lower alkane at high yield, in which the unsaturated aldehyde obtained as above is further oxidized at vapor phase in the presence of molecular oxygen to provide unsaturated acid.
More specifically, C2-C5 lower alkanes signify ethane, propane, n-butane, isobutane, n-pentane and isopentane. The catalysts of the present invention are used in oxidative dehydrogenation reactions of these lower alkanes to produce corresponding olefins, more specifically, ethylene from ethane, propylene from propane, n-butene from n-butane, isobutene from isobutane, n-pentene from n-pentane and isopentene from isopentane. These lower alkanes may be used either singly or as a mixture of more than one. The oxidative dehydrogenation catalysts of the present invention are useful for the production of, in particular, propylene and isobutene from propane and isobutane, respectively.
Referring to the general formula (I), the catalysts in which, when xcex1=1, xcex2=0.02-2, and xcex3=0-1 are particularly preferred.
For improving the selectivity for, and yield of, the product, the catalysts of the general formula (I) in which E1 component is P, Sb, B, S, Nb, W or Re and E2 component is Cr, Fe, Sn, Na, Mg or Ce are preferred.
The oxidative dehydrogenation catalysts of general formula (I) of the present invention may be used as supported on a refractory inorganic carrier for the purpose of improving activity level and physical durability. As the refractory inorganic carrier, those generally used in preparation of this type of catalysts can be used, the representative examples thereof including silica, alumina, titania, zirconia, silica-alumina, silica-titania and silica-zirconia. In particular, silica and silica-alumina are preferred, because they give higher yield of object products. The ratio of silica in the silica-alumina catalyst system normally ranges from 10% by weight to less than 100% by weight. The amount of the catalytically active component to be carried is normally between 10 and 90% by weight of the refractory inorganic carrier.
The method of preparation of the oxidative dehydrogenation catalysts of the present invention is not subject to any critical limitations, but any of conventionally practiced methods or known methods for preparation of this type of catalysts can be used. For example, the catalysts may be prepared by the procedures comprising adding to a slurry of manganese dioxide powder antimony trioxide powder and aqueous solutions of phosphoric acid, boric acid, ammonium sulfate, telluric acid, ammonium chloride, niobium oxalate, ammonium tungstate, rhenium oxide and copper nitrate, etc. as E1 component; if necessary further adding aqueous solution of at least one element selected from the E2 component; further if necessary adding a carrier such as silica, alumina or the like thereto; condensing the mixture under heating with agitation for a prescribed period, drying the resultant paste at 80-300xc2x0 C.; pulverizing and molding the same; if necessary further crushing the same for size adjustment or re-drying at 80-300xc2x0 C.; and if necessary further firing the dry product at 300-800xc2x0 C. The firing atmosphere is subject to no limitation, and the firing may be conducted in air, an atmosphere of high or low oxygen concentration, a reducing atmosphere, in an inert gas such as nitrogen, helium, argon or the like, or in vacuum. In most desirable practice, the catalyst is not fired at the high temperatures but is contacted with the reaction gas containing the alkane or alkanes and oxygen as it has undergone the drying treatment or treatments at not higher than 300xc2x0 C. In that occasion, the reaction may be started at a temperature not lower than the prescribed level by way of a pretreating reaction, or directly at the prescribed temperature. In the latter case changes in catalytic activity may be observed at the initial stage of the reaction, but normally a stable activity level is reached within an hour.
The starting materials for catalyst preparation are not critical, but may be any of nitrate, sulfate, oxide, hydroxide, chloride, carbonate, acetate, oxygen acid, ammonium salt of oxygen acid, etc. of the elements.
As Mn source, besides powders of various oxides thereof or molded products which are useful as they are, manganese hydroxide slurries obtained upon treating an aqueous solution of, eg., manganese nitrate, with aqueous ammonia or the like are conveniently used. Any means used for catalyst preparation in general, for example, co-precipitation of a manganese compound with compounds of other additive elements from their aqueous solution, are applicable. As sulfur source, aqueous sulfuric acid or ammonium sulfate may be used, or the whole or a part thereof may be introduced in the form of sulfate(s) of other additive element(s). Similarly, halogen may be introduced as aqueous hydrogen halide or ammonium halide, or in the form of halide(s) of other additive element(s).
Again the use form of refractory inorganic carrier is subject to no critical limitation, which allows versatile selection according to the form of use of the catalyst, such as, besides molded products, powder of oxide or hydroxide, gel or sol.
The starting gas to be subjected to the vapor phase oxidative dehydrogenation reaction according to the present invention may if necessary contain a diluent gas, besides lower alkane(s) and molecular oxygen. As the molecular oxygen, air or pure oxygen is used, normally at a ratio of 0.1-5 mols per mol of alkane. As the diluent gas, an inert gas such as nitrogen, helium or carbon dioxide or steam is conveniently used.
The reaction conditions for carrying out the vapor phase oxidative dehydrogenation of the present invention are subject to no critical limitation. For example, the starting gas as described above is contacted with an oxidative dehydrogenation catalyst of the present invention under such conditions as: at a space velocity of 300-30,000 hrxe2x88x921 at a temperature between 250 and 650xc2x0 C. While the reaction is normally conducted under atmospheric pressure, a reduced or elevated pressure may be used. The reaction system again is not critical, which may be a fixed bed system, moving bed system or fluidized bed system. It may also be one-pass system or recycling system.
The olefines (alkenes) which are obtained through the vapor phase oxidative dehydrogenation of C2-C5 lower alkanes (alkane oxidative dehydrogenation step) using the catalyst of the present invention can be further oxidized to produce unsaturated aldehydes and unsatuated acids (alkene oxidation step). The unsaturated aldehydes can further be oxidized to produce unsaturated acids (aldehyde oxidation step). Thus formed unsaturated aldehydes and/or unsaturated acids are trapped with an absorption column (absorbing step). As the oxygen source in the present invention, air and/or oxygen produced by such methods as cryogenic method, P.S.A. (pressure swing adsorption) method and the like can be used. According to the present invention, it is possible to form from lower alkanes the corresponding olefins, without side-production of hydrogen. If necessary oxygen and/or steam may be added to the gases to be introduced in each of the above steps, and such additional oxygen and/or steam are supplied by, for example, air, above-described oxygen, water and/or the gas discharged of said absorbing step.
As one specific example of useful catalyst in the alkene oxidation step, those expressed by following general formula (2) may be named:
MoaBibFecAdBeCfDgOxxe2x80x83xe2x80x83(2)
in which Mo is molybdenum; Bi is bismuth; Fe is iron; A is at least one element selected from the group consisting of cobalt and nickel; B is at least one element selected from the group consisting of alkali metals and thallium; C is at least one element selected from the group consisting of silicon, aluminium, zirconium and titanium; D is at least one element selected from the group consisting of tungsten, phosphorus, tellurium, antimony, tin, cerium, lead, niobium, manganese, arsenic and zinc; and O is oxygen: and the ratio of those elements is, when a=12, b=0.1-10, c=0.1-20, d=2-20, e=0.001-10, f=0-30, g=0-4 and x is a numerical value determined by the state of oxidation of those elements other than oxygen.
Also as one specific example of useful catalyst in the aldehyde oxidation step, those expressed by following general formula (3) may be named:
MohViWjEkFlGmHnOxxe2x80x83xe2x80x83(3)
in which Mo is molybdenum; V is vanadium; W is tungsten; E is at least one element selected from the group consisting of copper, cobalt, bismuth and iron; F is at least one element selected from the group consisting of antimony and niobium; G is at least one element selected from the group consisting of silicon, aluminium, zirconium and titanium; H is at least one element selected from the group consisting of alkaline earth metals, thallium, phosphorus, tellurium, tin, cerium, lead, manganese and zinc; and O is oxygen: and the ratio of those elements is, when h=12, i=0.1-10,j=0-10, k=0.1-20, l=0-10, m=0-10, n=0-30, and x is a numerical value determined by the state of oxidation of those elements other than oxygen.
The lower alkane oxidative dehydrogenation catalysts according to the present invention excel in the oxidative dehydrogenation ability and enable the production from lower alkanes of corresponding olefins at high yield.
Furthermore, due to their higher activity level than that of known catalyst system, the amount of the catalyst necessary for securing the same level of STY (space time yield) is far less than that of conventional catalysts, such as from ⅓ to {fraction (1/10)}.
Also according to the present invention, unsaturated aldehyde and/or unsaturated acid can be produced from lower alkanes stably at high yield.